Functional analysis of CSB in telomere maintenance and DNA double-strand break repair

Abstract

Cockayne syndrome (CS) is a rare, segmental premature aging disorder in which the majority of cases are caused by mutations in the Cockayne syndrome group B protein (CSB). CSB is a multifunctional protein implicated in DNA repair, transcription and chromatin remodeling. The results presented here demonstrate that CSB plays an important role in telomere maintenance and DSB repair. We find that CS cells accumulate telomere doublets, have increased telomere-bound TRF1, decreased TERRA levels and a defect in telomerase-dependent telomere lengthening. These results imply that CS patients may be defective in telomere maintenance. We also uncover a novel and important role of CSB in DNA DSB repair. We show that CSB facilitates HR and supresses NHEJ during S and G2 phase. We find that CSB interacts with RIF1 and is recruited by RIF1 to DSBs in S phase. At DSBs, CSB remodels the chromatin extensively, which in turn limits RIF1 recruitment and promotes BRCA1 accumulation. The chromatin remodeling activity of CSB requires not only damage-induced phosphorylation on S10 by ATM but also cell cycle-dependent phosphorylation of S158 by cyclin A-CDK2. Both modifications are needed for the intramolecular interaction of CSB N-terminal domain with its ATPase domain. This intramolecular interaction has previously been reported to regulate the ATPase activity of CSB. Taken together, these results suggest that ATM and CDK2 control of CSB to promote chromatin remodeling, which in turn inhibits RIF1 in DNA DSB repair pathway choice.